Detect recursively referencing requirements files

news/12653.feature.rst

@@ -0,0 +1,2 @@
+Detect recursively referencing requirements files and help users identify
+the source.

src/pip/_internal/req/req_file.py

@@ -324,11 +324,15 @@ def __init__(
     ) -> None:
         self._session = session
         self._line_parser = line_parser
+        self._parsed_files: dict[str, Optional[str]] = {}
 
     def parse(
         self, filename: str, constraint: bool
     ) -> Generator[ParsedLine, None, None]:
         """Parse a given file, yielding parsed lines."""
+        self._parsed_files[os.path.abspath(filename)] = (
+            None  # The primary requirements file passed
+        )
         yield from self._parse_and_recurse(filename, constraint)
 
     def _parse_and_recurse(
@@ -353,11 +357,25 @@ def _parse_and_recurse(
                 # original file and nested file are paths
                 elif not SCHEME_RE.search(req_path):
                     # do a join so relative paths work
-                    req_path = os.path.join(
-                        os.path.dirname(filename),
-                        req_path,
+                    # and then abspath so that we can identify recursive references
+                    req_path = os.path.abspath(
+                        os.path.join(
+                            os.path.dirname(filename),
+                            req_path,
+                        )
                     )
-
+                if req_path in self._parsed_files:
+                    initial_file = self._parsed_files[req_path]
+                    tail = (
+                        f" and again in {initial_file}"
+                        if initial_file is not None
+                        else ""
+                    )
+                    raise RequirementsFileParseError(
+                        f"{req_path} recursively references itself in {filename}{tail}"
+                    )
+                # Keeping a track where was each file first included in
+                self._parsed_files[req_path] = filename
                 yield from self._parse_and_recurse(req_path, nested_constraint)
             else:
                 yield line

tests/unit/test_req_file.py

@@ -1,6 +1,7 @@
 import collections
 import logging
 import os
+import re
 import textwrap
 from optparse import Values
 from pathlib import Path
@@ -345,6 +346,63 @@ def test_nested_constraints_file(
         assert reqs[0].name == req_name
         assert reqs[0].constraint
 
+    def test_recursive_requirements_file(
+        self, tmpdir: Path, session: PipSession
+    ) -> None:
+        req_files: list[Path] = []
+        req_file_count = 4
+        for i in range(req_file_count):
+            req_file = tmpdir / f"{i}.txt"
+            req_file.write_text(f"-r {(i+1) % req_file_count}.txt")
+            req_files.append(req_file)
+
+        # When the passed requirements file recursively references itself
+        with pytest.raises(
+            RequirementsFileParseError,
+            match=(
+                f"{re.escape(str(req_files[0]))} recursively references itself"
+                f" in {re.escape(str(req_files[req_file_count - 1]))}"
+            ),
+        ):
+            list(parse_requirements(filename=str(req_files[0]), session=session))
+
+        # When one of other the requirements file recursively references itself
+        req_files[req_file_count - 1].write_text(
+            # Just name since they are in the same folder
+            f"-r {req_files[req_file_count - 2].name}"
+        )
+        with pytest.raises(
+            RequirementsFileParseError,
+            match=(
+                f"{re.escape(str(req_files[req_file_count - 2]))} recursively"
+                " references itself in"
+                f" {re.escape(str(req_files[req_file_count - 1]))} and again in"
+                f" {re.escape(str(req_files[req_file_count - 3]))}"
+            ),
+        ):
+            list(parse_requirements(filename=str(req_files[0]), session=session))
+
+    def test_recursive_relative_requirements_file(
+        self, tmpdir: Path, session: PipSession
+    ) -> None:
+        root_req_file = tmpdir / "root.txt"
+        (tmpdir / "nest" / "nest").mkdir(parents=True)
+        level_1_req_file = tmpdir / "nest" / "level_1.txt"
+        level_2_req_file = tmpdir / "nest" / "nest" / "level_2.txt"
+
+        root_req_file.write_text("-r nest/level_1.txt")
+        level_1_req_file.write_text("-r nest/level_2.txt")
+        level_2_req_file.write_text("-r ../../root.txt")
+
+        with pytest.raises(
+            RequirementsFileParseError,
+            match=(
+                f"{re.escape(str(root_req_file))} recursively references itself in"
+                f" {re.escape(str(level_2_req_file))}"
+            ),
+        ):
+            list(parse_requirements(filename=str(root_req_file), session=session))
+
     def test_options_on_a_requirement_line(self, line_processor: LineProcessor) -> None:
         line = (
             'SomeProject --global-option="yo3" --global-option "yo4" '